1. Field of the Invention
The present invention relates to the real-time, optical writing and reading of high density information and more particularly to methods for such writing and reading.
2. Description of the Prior Art
In general, high density optical storage media carry information in the form of tracks of small (usually on the order of a micron or less in size), optically-detectable marks formed in the surface of a substrate or in thin material layer(s) deposited on a substrate. Information is recovered (read) by scanning the tracks with a tightly focussed spot of light., e.g., from a laser. The recovered information is in the form of a fluctuating electrical signal obtained from a photodetector that senses the read-out light after modulation by the track markings.
There have been a variety of approaches for achieving the general functions outlined above. Although there are others, the most popular storage media format has been a disc with a single spiral track or concentric circular tracks. For convenience, this discussion will refer to the storage media as optical discs, with the understanding that other formats are, in general, equivalent as to utility with the present invention.
With regard to record formation, i.e., recording or writing on optical discs, approaches can be divided as: (1) real-time discs (ones ready for reading immediately after writing) or (2) processed discs (ones requiring further processing after recording before they can be read). Typical of the prior art real-time disc type are heat-deformable elements comprising a substrate bearing a very thin metal or dye layer that is deformed (e.g., displaced or ablated) by the heat generated from an absorbed writing laser beam, which is modulated in intensity according to the signal to be recorded. Typical of the processed disc type are ones formed by: (1) recording exposure of a photosensitive material, such as positive photoresist; (2) chemical development of that material to form a relief pattern and (3) metallization of the relief pattern.
With regard to reading approach, the optical discs can, in one manner, be classified as being of a transmissive or reflective type, depending on whether the reading light beam passes completely through the disc to a detector on the opposite side or is reflected from the disc to a detector on the same side. The reflective type offers potential simplicity by allowing a single lens to both focus the reading beam on the disc and collect the modulated light returned from the disc.
A more subtle distinction of disc read out approaches can be made based on the type of predominating interaction between the reading light spot and the recorded marks on the disc. Thus, approaches that obtain a signal based on differences in the electric field amplitude of reading light leaving the marked and unmarked portions of the disc (e.g., because of optical density variations) can be classified as amplitude variation systems. Systems of this type can be written in real time or be processed. In distinction, approaches which obtain a signal based on differences or transitions in the phase of reading light leaving marked and non-marked portions of a disc can be characterized as phase shift systems. Systems of this type heretofore have not readily been formed in real time.
Undoubtedly there are pros and cons with respect to the desirability of the different aspects described above, particularly when it is considered that there are a variety of applications or end uses for such discs. One of the most challenging applications thus far identified for such discs is as a low-error-rate, high-density, real-time, data storage medium. One purpose of the present invention is to improve optical disc technology to better meet such challenging applications.
In such applications, and others, e.g., real time TV signal recording, information usually is encoded in the relative placement of discrete marks along a track. Thus any process which alters or impairs detection of the proper spatial frequency of the recorded marks will represent a potential noise (error) source in the demodulated electrical playback signal. At least two such noise components are directly related to the optical disc structure. The first is related to the precision with which the individual marks can be placed on the optical disc. Any variation in geometry of identically exposed marks, recorded along the signal track, will represent a "frozen in", materials-associated error. The second is related to the contrast of read-out signal obtainable from the optical disc. That is, the contrast of the light fluctuations that occur at the detector as the playback spot scans across recorded marks should be as high as possible to minimize the photon shot noise associated with optical detection. This contrast is influenced by the optical properties of the disc.